In this undated file photo provided by the US army, Chelsea Manning poses for a photo wearing a wig and lipstick. Photograph: Uncredited/AP

A year after being sentenced to a 35-year prison term for giving secret documents to WikiLeaks, US army private Chelsea Manning says the military is continuing to deny her gender-reassignment treatment.

In a letter sent to NBC news and released on Friday, Manning says the Defence Department has not followed through with its promises after the defence secretary, Chuck Hagel, approved a treatment plan that includes allowing her to dress as a woman.

“Unfortunately, despite silence, and then lip-service, the military has not yet provided me with any such treatment,” Manning wrote in a statement sent to NBC from Fort Leavenworth prison in Kansas. “However, prisons – and especially military prisons – reinforce and impose strong gender norms – making gender the most fundamental aspect of institutional life.”

“Despite having received at least four diagnoses of gender dysphoria, Chelsea has received no treatment,” Manning’s attorney, David Coombs, told NBC News. Manning’s requests for hormone therapy and amended grooming and clothing standards have all been denied, Coombs said.

“If the military does not do the right thing, we are prepared to pursue litigation to vindicate her constitutional rights,” Coombs said. Earlier this month, Coombs threatened to file a lawsuit if treatment did not begin by 4 September.

Manning said in her letter to NBC that the prison’s gender norms make it hard to “feel comfortable in my own skin”.

“For example, in my daily life, I am reminded of this when I look at the name on my badge, the first initial sewed into my clothing, the hair and grooming standards that I adhere to, and the titles and courtesies used by the staff. Ultimately, I just want to be able to live my life as the person that I am.”

“The continued failure to provide Ms Manning with this treatment is inconsistent with well-established medical protocols and basic constitutional principles,” Chase Strangio, attorney for the ACLU’s Lesbian Gay Bisexual and Transgender Project, said in a statement to the Associated Press earlier this month.

Strangio said refusing to treat Manning is “cruel and unusual punishment,” and advocates for Manning stressed that “there is a clear medical consensus that gender dysphoria is a serious medical condition,” which, left untreated, “can lead to severe medical problems”.

The army declined to tell NBC if treatment would begin soon.

“The Department of Defence has approved a request by army leadership to provide required medical treatment for an inmate diagnosed with gender dysphoria. I can’t discuss the medical needs of an individual,” an army spokeswoman, Lt Col Alayne Conway, told NBC.

Calls from the Guardian to Fort Leavenworth were directed to the military’s chief public information office. Calls to that office were not immediately returned.